An electrocardiogram is a graphic representation of the electrical forces produced by the heart. It produces a diagnostic test which can be used by skilled and trained medical personnel to interpret various of the heart processes, sometimes identifying abnormal heart functioning and anatomy.
Numerous factors other than the heart itself can contribute to the record made. Among these are skin resistance, thickness of the chest, position of the heart in the thoracic cage, electrical interference, polarization, skeletal muscle tremors, infections, fear, shock, drugs, and many other factors. Included in the extrinsic factors is exercise.
Accordingly, and within limitations, a wide variety of exercise testing procedures have been employed in conjunction with electrocardiograms. The exercise stress test is a valuable part of cardiac evaluation. This test is performed with the patient exercising on a treadmill or bicycle ergometer and performing measured amounts of exercise of increasing intensity. During the test the heart rate, blood pressure and electrocardiogram are routinely recorded. The test ends at certain defined end points.
As is well known, the electrocardiograph waves have been correlated with the contracting heart. The P wave is associated with the atrial contraction, while the ventricular contraction is associated with the QRS complex and the T wave deflection. The septum and ventricles constituting the major muscle mass, are correspondingly responsible for the most important portion of the electrocardiogram. In addition, the atrial activity plays an important part in determining the basic rhythm of the heartbeat. The electrical impulse originates in the sinus node, travels to the atrionodal junction by way of internodal tracts as well as to the left atrium via interatrial tracts, next into the atrioventricular node and subsequently through the septum to the free ventricle walls. Sinus rhythm is characterized by recurrence of the P waves at regular intervals, followed in normal time sequence by QRS complexes and the T wave.
A normal electrocardiogram displays this sequence of waves on a chart or recorder paper, in some cases, or on a CRT display. Standardized equipment has produced uniformity in electrocardiograms, so that the chart is based on an X-Y coordinate system in which the isoelectric or reference base line travels from left to right on the graph, with standard 0.04 second intervals marked. Displacement above and below the isoelectric line is marked in one mm lines, so that the grid presents squares which are one mm or 0.04 seconds on a side.
The electrocardiogram reports the activity of the heart in terms of electric currents. Each mechanical contraction, atrial or ventricular, is associated with two electrical processes. The first process is activation or depolarization, during which the electrical charges on the surface of the muscle cell change from positive to negative. Repolarization, or return to the resting state, then follows with resultant replacement of the positive surface charges. Depolarization of the ventricles is a rapid affair, represented electrocardigraphically by the QRS complex; repolarization of the ventricles is slower and is designated by the T wave. The S-T segment represents the period when all parts of the ventricles are in the depolarized state. The S-T segment begins at the J-point and ends at the beginning of the T wave. Beyond this basic description, countless volumes have been written on the truly amazing amount of information which can be learned from various tests associated with electrocardiograms.
As previously mentioned, exercise, and exercise testing, is one factor which influences the electrocardiogram. A normal resting electrocardiogram can be obtained even though the patient has coronary atherosclerotic heart disease. In patients with angina pectoris, the resting electrocardiogram may be normal in about 70% of cases.
Exercise tests are only to be performed under the supervision of and in the presence of a physician who will monitor the tests and, of course, stop them immediately on signs of heart difficulties for the patient. Even when no physical symptoms are noticed by the patient, either due to insensitivity, preoccupation with test, nervousness, or whatever, heart damage can be occurring during the test and therefore the physician is highly anxious to monitor the test.
A most important electrocardiographic criteria of a positive response to the exercise test involve S-T segment changes. As the heart rate increases with exercise, the oxygen requirement of the heart muscle increases. The lack of oxygen to heart muscle as it is needed is the hallmark of coronary heart disease. If the coronary arteries get narrowed for whatever reason, they still might supply enough blood for a heart at rest rate, such as 60 or 70 beats per minute. But, as the rate increases to 120, or 130 or higher, the heart no longer receives enough oxygen in the blood. At that point, changes occur in the cardiac complex. Most meaningful is the S-T segment change, which is considered the hallmark of coronary disease and has the most diagnostic significance.
One of the most significant techniques of exercise testing is to accelerate the heart rate under controlled conditions and review electrocardiogram results, particularly the S-T segment, to see if the slope becomes more horizontal and depressed. During the testing, it should be, and is, kept in mind that evaluation of the S-T segment as it deviates from normalcy must be done at the time of the test, quickly and accurately, so that prompt medical care can be instituted if needed. Later analysis of the electrocardiograms in great detail and under suitable conditions to derive the most knowledge is of course one of the goals of the test. Primary, on sight analysis of the changes in S-T segments, must be done however, to give a first, safe diagnosis of the test. Because of this, quick accurate reading and evaluation of the normalcy of an S-T segment of an electrocardiogram is essential.